| Regional carbon cycle is the core content of global carbon cycle research.Southwest China has a wide forest coverage area,which is affected by the Qinghai Tibet Plateau.This region is sensitive to global climate change.Its vegetation diversity has a significant impact on soil organic carbon density and carbon cycle.It is necessary to correctly evaluate the forest organic carbon density in Southwest China and grasp its spatial and temporal distribution characteristics.The ecological environment of the region and the measures of adjusting forest carbon sink have important guiding significance.However,there are still uncertainties in the assessment of forest soil organic carbon density and the dominant factors of soil carbon cycle parameters in this region,which need further study.Therefore,this study collects and collates 204 pieces of 649 data from 1990 to 2019 and publishes relevant literature on the density of forest soil organic carbon in Southwest China at home and abroad.Combined with the field soil investigation of typical ecosystems in Southwest China,taking organic carbon density and organic carbon content as main parameters.The spatial-temporal distribution pattern of soil carbon cycle parameters in Southwest China is explored.The main research results are as follows:(1)The average SOC density of forest soil in Southwest China was 11.83 kg/m~2.For different forest types,the SOC density of coniferous forest was significantly higher than that of broad-leaved forest and mixed broadleaf-conifer forest(P<0.05).Regression analysis showed that coniferous forest and broad-leaved forest increased linearly with the increase of forest age,and soil organic carbon density and forest age of coniferous and mixed broadleaved forest increased first and then decreased,and a turning point appeared near the stand age of 80 a.There was no significant correlation between soil organic carbon density and time in different forest types(P>0.05).The average SOC content of forest soil in Southwest China was 27.83 g/kg,the highest content of organic carbon was found in the mixed forest(31.53 g/kg).But there was no significant correlation between soil organic carbon content among different forest types(P>0.05).Regression analysis showed that Southwest China forest soil organic carbon content increased linearly with the increase of time,and soil organic carbon content and forest age of coniferous forest increased first and then decreased,and a turning point appeared near the stand age of 100 a.(2)The density of forest soil organic carbon in Southwest China changed significantly with longitude,latitude and altitude.The SOC density showed a quadratic relationship with longitude and a turning point appeared near 105 °E.Moreover,with the increase of latitude and altitude,there showed a linear increase trend.The SOC density of different forest types increased linearly with the increase of altitude,but there was no correlation with latitude.There is a single peak curve between broad-leaved forest and longitude,and coniferous forest shows a decreasing trend with the increase of latitude.The density of forest soil organic carbon content in Southwest China changed significantly with longitude,latitude and altitude(P<0.05).The soil organic carbon content decreased linearly with longitude and increased linearly with latitude and altitude(P<0.05).And all forest types increased linearly with the altitude.The content of soil organic carbon in coniferous forest decreased linearly with longitude(P<0.05),but the content of soil organic carbon in coniferous forest increased linearly with longitude and altitude(P<0.05).(3)The average organic carbon density of natural forest and artificial forest in Southwest China is 13.59 kg/m~2 and 10.42 kg/m~2,respectively.For natural and plantation forest,the SOC density of the three different forest types was: coniferous forest > mixed broadleaf-conifer forest > broad-leaved forest.There was significant difference in soil organic carbon content between artificial and natural forest in Southwest China(P<0.05).The average organic carbon content of natural forest and artificial were 35.75 g/kg and 22.53 g/kg,respectively.For the soil organic carbon content of natural forest,the highest value is 43.85 g/kg for coniferous forest,and 23.58 g/kg for artificial forest.(4)SOC density of forest soil in Southwest China decreased significantly with the increase of annual average temperature,which had no correlation with annual rainfall and relative annual humidity.In addition,the soil organic carbon density of different forest types showed a significant decreased trend with the increase of annual average temperature,which had no correlation with annual rainfall.Among the three forest types,only broad-leaved forest had a good linear relationship with relative annual humidity,which increased with the increase of air relative humidity.The organic carbon content of forest soil decreased linearly with the increase of annual average temperature.However,the organic carbon content of forest soil in Southwest China presents a gentle quadratic function curve with the annual rainfall,but there is no significant correlation with the air relative humidity(P>0.05).The content of soil organic carbon in broad-leaved forest decreased linearly with the increase of annual average temperature(P<0.05),and increased significantly with the increase of air relative humidity(P<0.05).Soil organic carbon content of coniferous forest decreased linearly with the increase of annual average temperature and annual rainfall.There is a U-shaped curve between soil organic carbon content and annual average temperature,and the turning point is near 14 ?.(5)The SOC density of forest soil in Southwest China showed different changes with the change of soil physical and chemical properties.The increase of soil total nitrogen increased linearly(P<0.05),but had no correlation with soil total phosphorus and pH.Among them,broad-leaved forest decreased linearly with the increase of pH,coniferous forest increased linearly with the increase of soil total nitrogen.The organic carbon content of forest soil in Southwest China increased with the increase of total nitrogen and total phosphorus.There was no correlation between soil pH and soil pH(P>0.05).The content of soil organic carbon in broad-leaved forest increased with the increase of total nitrogen and total phosphorus(P<0.05),and decreased linearly with the increase of pH.There was no correlation between soil organic carbon content and pH(P>0.05).However,for coniferous and broad-leaved mixed forest,there was a quadratic function curve between soil organic carbon content and soil total nitrogen.With the decrease of total nitrogen first and then increase,the turning point was around 5 g/kg(P<0.05),which was not significant with the change of soil total phosphorus and pH(P>0.05).(6)Through the analysis of equation structure model,the density of soil organic carbon in Southwest China is mainly controlled by forest type,forest age,pH and soil nitrogen.The main controlling factor was soil nitrogen,which had significant positive effect on soil organic carbon density(0.35,P<0.001).pH was the secondary controlling factor of soil organic carbon,and had significant negative effect on soil organic carbon density(-0.23,P<0.001).Forest age had a significant positive effect on soil organic carbon density(0.17,P<0.001).The results of structural equation model show that soil organic carbon content of forest in Southwest China is mainly controlled by soil factor,climate factor and stand factor,which explains 56% of variation of soil organic carbon content.The main controlling factor was soil nitrogen,and its path coefficient was 0.61(P<0.001).Temperature and pH had significant negative effects on soil organic carbon content,and path coefficients were-0.20 and-0.16(P<0.05).To sum up,the forest in Southwest China has great carbon sink potential,but soil organic carbon has no obvious change with time,with the change of forest age,and has obvious three-dimensional zonality.Soil physical and chemical properties and hydrothermal environment affect the content of soil organic carbon density of different forest types.In addition,soil organic carbon density and accumulated carbon content in Southwest China are mainly affected by soil nitrogen,and to some extent by climate and forest factors.These results are helpful for us to provide the basis for the estimation of regional forest soil carbon storage,and to construct and provide reference for the further study of forest soil carbon cycle parameters and carbon sink function in different regions.However,in order to further understand the process of regional forest soil carbon cycle and reveal the capacity of regional forest soil carbon sequestration,it may be necessary to further study the spatialtemporal pattern and driving mechanism of soil carbon components(soil organic carbon mineralization,refractory carbon,inorganic carbon,soluble organic carbon,oxidizable organic carbon,microbial biomass carbon,etc.)in the future parameter system construction. |
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